Various methods of electrically connecting components such as connectors and integrated circuit devices to printed boards are well known in the art. Surface mount technology is increasingly being employed as cost-effective method. Examples of surface mount technology include pin grid arrays (PGA's), ball grid arrays (BGA's), column grid arrays (CGA's), and land grid array (LGA's). Generally with such methods, a solder paste is introduced between surfaces to be joined, the solder paste reflowed, and then cooled to form a mechanical and electrically conductive joint.
As the name implies, ball grid arrays (BGA's) utilize a grid or array of solder bumps or balls arranged on one side of an electrical component to effectuate electrical and mechanical connection with a printed board. The ball attachment sites are typically referred to as lands or pads. Solder past is disposed on the lands (and/or on the balls), such as through screening or masking techniques, and the electrical component is then positioned on the board so that the balls and lands are registered. The board is then processed at an elevated temperature sufficient to cause the solder and at least a portion or all of the solder ball to flow and fuse with the underlying land/pad to form the electrical interconnection. See, for example, U.S. Pat. No. 6,325,644 assigned to the assignee of this patent, and patents within that patent, all of which is incorporated by reference herein.
Locator pins extending from the electrical component may be employed to facilitate the initial alignment of the electrical component. The locator pins are inserted into through holes located in the board at locations along the periphery of the array of lands/pads. Pin diameters may be slightly larger than the corresponding through hole diameters so that an interference fit is achieved. Alternatively, the pin diameters may be smaller than the through hole diameters. In this scenario, adhesive or solder paste may be applied in and around the through holes to help retain the electrical component on the substrate.
By selecting locator pins with a relatively smaller diameter and applying solder within the through holes, the BGA balls are allowed to float (that is, not rigidly restricted) during a reflow process resulting in self-alignment and ultimately a low stress solder joint. Component retention however may be compromised with this configuration when the board is manipulated before the reflow heat is substantially dissipated, or when the board is rotated so that additional electrical components can be mounted on other available surface areas with subsequent reflow steps.
Accordingly, there is a need for a method of mounting electrical components on a substrate that provides for a low stress solder joint while effectively retaining the mounted components during ensuing double-sided handling and/or processing during a second reflow step.